1. Field of the Invention
The invention relates an extreme ultraviolet radiation transparent structure in a vacuum wall. More particularly, the invention relates to the use of the transparent structure in a lithographic projection apparatus comprising:
an illumination system constructed and arranged to supply a projection beam of radiation;
a mask table constructed to hold a mask;
a substrate table constructed to hold a substrate; and
a projection system constructed and arranged to image an irradiated portion of the mask onto a target portion of the substrate.
2. Discussion of Related Art
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics and catadioptric systems, for example. In addition, the first and second object tables may be referred to as the xe2x80x9cmask tablexe2x80x9d and the xe2x80x9csubstrate tablexe2x80x9d, respectively. Further, the lithographic apparatus may be of a type having two or more mask tables and/or two or more substrate tables. In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more stages while one or more other stages are being used for exposures. Twin stage lithographic apparatus are described in International Patent Applications WO 98/28665 and WO 98/40791, for example.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the mask (reticle) may contain a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target area (comprising one or more dies) on a substrate (silicon wafer) which has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target areas which are successively irradiated via the mask, one at a time. In one type of lithographic projection apparatus, each target area is irradiated by exposing the entire mask pattern onto the target area in one go; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus, which is commonly referred to as a step-and-scan apparatus, each target area is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally less than 1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned from International Patent Application WO 97/33205.
In a lithographic apparatus the size of features that can be imaged onto the substrate is limited by the wavelength of the projection radiation. To produce integrated circuits with a higher density of devices, and hence higher operating speeds, it is desirable to be able to image smaller features. Whilst most current lithographic projection apparatus employ ultraviolet light generated by mercury lamps or excimer lasers, it has been proposed to use shorter wavelength radiation of around 13 nm. Such radiation is termed extreme ultraviolet radiation, also referred to as XUV or EUV radiation. XUV generally refers to the wavelength range from several tenths of nanometer to several tens of nanometers, combining the soft x-ray and vacuum UV range, whereas EUV is normally used in conjunction with lithography (EUVL) and refers to a radiation band from approximately 5 to 20 nm, i.e. part of the XUV range.
Possible sources include, for instance, laser-produced plasma sources, discharge plasma sources, or synchrotron radiation from electron storage rings. An outline design of a lithographic projection apparatus using synchrotron radiation is described in xe2x80x9cSynchrotron Radiation Sources and Condensers for Projection X-Ray Lithographyxe2x80x9d, J. B. Murphy et al, Applied Optics, Vol. 32, No. 24, pp. 6920-6929 (1993). Apparatus using discharge plasma sources are described in: W. Partlo, I. Fomenkov, R. Oliver, D. Birx, xe2x80x9cDevelopment of an EUV (13.5 nm) Light Source Employing a Dense Plasma Focus in Lithium Vaporxe2x80x9d, Proc. SPIE 3997, pp. 136-156 (2000); M. W. McGeoch, xe2x80x9cPower Scaling of a Z-pinch Extreme Ultraviolet Sourcexe2x80x9d, Proc. SPIE 3997, pp. 861-866 (2000); W. T. Silfvast, M. Klosner, G. Shimkaveg, H. Bender, G. Kubiak, N. Fornaciari, xe2x80x9cHigh-Power Plasma Discharge Source at 13.5 and 11.4 nm for EUV lithographyxe2x80x9d, Proc. SPIE 3676, pp. 272-275 (1999); and K. Bergmann et al., xe2x80x9cHighly Repetitive, Extreme Ultraviolet Radiation Source Based on a Gas-Discharge Plasmaxe2x80x9d, Applied Optics, Vol. 38, pp. 5413-5417 (1999). So-called xe2x80x9cundulatorsxe2x80x9d and xe2x80x9cwigglersxe2x80x9d have been proposed as an alternative source of extreme ultraviolet radiation. In these devices, a beam of electrons traveling at high, usually relativistic, speeds, e.g. in a storage ring, is caused to traverse a series of regions in which magnetic fields perpendicular to the beam velocity are established. The directions of the magnetic field in adjacent regions are mutually opposite, so that the electrons follow an undulating path. The transverse accelerations of the electrons following the undulating path cause the emission of Maxwell radiation perpendicular to the direction of the accelerations, i.e. in the direction of the non-deviated path.
Radiation sources may require the use of a rather high partial pressure of a gas or vapor to emit XUV radiation, such as discharge plasma radiation sources referred to above. In a discharge plasma source a discharge is created in between electrodes, and a resulting partially ionized plasma is subsequently caused to collapse to yield a very hot plasma that emits radiation in the XUV range. The very hot plasma is quite often created in Xe, since a Xe plasma radiates in the EUV range around 13.5 nm. For an efficient EUV production, a typical pressure of 0.1 mbar is required near the electrodes of the radiation source. A drawback of having such a rather high Xe pressure is that Xe gas absorbs EUV radiation. For example, 0.1 mbar Xe transmits over 1 m only 0.3% EUV radiation having a wavelength of 13.5 nm. It is therefore required to confine the rather high Xe pressure to a limited region around the source. To reach this the source can be contained in its own vacuum chamber that is separated by a chamber wall from a subsequent vacuum chamber in which the collector mirror and illumination optics may be contained. However, an EUV radiation transparent opening is needed to pass the EUV radiation emitted by the source to the next vacuum chamber. Since a large opening in the wall, required to collect sufficient EUV radiation, would cause an elevated pressure in the next vacuum chamber, the opening might be closed off using a thin window of a few micron thickness or less, which is (partially) transparent for EUV radiation. Such a thin window will, however, not survive the heat load from the high-power EUV radiation source that is needed for EUV lithography.
It is an object of the present invention is to provide a structure in a vacuum chamber wall that is transparent for EUV radiation and further presents a gas barrier so as to be able to maintain different vacuum levels in vacuum chambers on both sides of the vacuum chamber wall.
Further objects of the invention will become apparent from the description of the invention that follows.
According to a first aspect of the present invention there is provided a lithographic projection apparatus comprising an illumination system constructed and arranged to supply a projection beam of radiation; a mask table constructed to hold a mask; a substrate table constructed to hold a substrate; and a projection system constructed and arranged to image an irradiated portion of the mask onto a target portion of the substrate; and further comprising two vacuum chambers separated by a chamber wall incorporating a channel structure comprising adjacent narrow channels separated by walls that are substantially parallel to a propagation direction of said radiation so as to pass said radiation from one of said vacuum chambers to the other one, said propagation direction being substantially along an optical axis of said apparatus.
According to a second aspect of the invention there is provided an optical apparatus comprising a radiation source constructed to generate a beam of radiation; and two vacuum chambers separated by a chamber wall incorporating a channel structure comprising adjacent narrow channels separated by walls that are substantially parallel to a propagation direction of said radiation so as to pass said radiation from one of said vacuum chambers to the other one, said propagation direction being substantially along an optical axis of said apparatus.
According to yet a further aspect of the invention there is provided a method of manufacturing a device using a lithographic projection apparatus comprising an illumination system constructed and arranged to supply a projection beam of radiation; a mask table constructed to hold a mask containing a mask pattern; a substrate table constructed to hold a substrate that is at least partially covered by a layer of radiation-sensitive material; and a projection system constructed and arranged to image an irradiated portion of the mask onto a target portion of the substrate, and further comprising two vacuum chambers separated by a chamber wall incorporating a channel structure comprising adjacent narrow channels separated by walls that are substantially parallel to a propagation direction of said radiation so as to pass said radiation from one of said vacuum chambers to the other one, said propagation direction being substantially along an optical axis of said apparatus, said method comprising the step of using the projection beam of irradiation to project an image of at least a portion of the mask pattern onto a target portion on the substrate.
In a manufacturing process using a lithographic projection, a pattern in a mask is imaged onto a substrate which is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
Although specific reference has been made hereabove to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget areaxe2x80x9d, respectively.
Further, this description concentrates on lithographic apparatus and methods employing a mask to pattern the radiation beam entering the projection system and it should be noted that the term xe2x80x9cmaskxe2x80x9d should be taken in a broad context of lithographic apparatus and methods. xe2x80x9cMaskxe2x80x9d should be interpreted as generally referring to generic xe2x80x9cpatterning meansxe2x80x9d to pattern the said radiation beam. The terms xe2x80x9cmaskxe2x80x9d and xe2x80x9cpatterning meansxe2x80x9d as here employed refer broadly to means that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term xe2x80x9clight valvexe2x80x9d has also been used in this context. Generally, the said pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device. The term xe2x80x9cmask tablexe2x80x9d should be broadly interpreted as any means for holding the xe2x80x9cpatterning meansxe2x80x9d. Besides a mask plate or reticle on a mask table, such patterning means include the following exemplary embodiments:
a programmable mirror array. An example of such a device is an addressable surface having a control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the said undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the addressable surface. The required matrix addressing can be performed using suitable electronic means. More information on such mirror arrays can be gleaned, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, which are incorporated herein by reference; and
a programmable LCD array. An example of such a construction is given in U.S Pat. No. 5,229,872, which is incorporated herein by reference.